<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        // 将div#demo.demo转化成<div id="demo" class="demo"></div>
        const str = "div#demo.demo"
        const reg = /^(div)#(demo)\.\2$/
        const str1 = str.replace(reg, function (match, $1, $2) {
            return "<" + $1 + " id=\"" + $2 + "\" class=\"" + $2 + "\"><\/" + $1 + ">"
        })
        console.log(str1);

    </script>

</body>

</html>